In a mixture in which solid bismaleimide reactants and liquid co-reactants are mixed, one problem often encountered by the application of heat is the imposition of thermal history on the subject resin and increasing viscosity, which is usually undesirable for composite and adhesive applications. Thermal history refers to the heat that is applied during processing to make the resin mixture. The resin mixture is the form of the product that contains all the reactants before the hardening that forms the final product. The resin mixture can then be applied to fibers to become prepreg which is then molded to form the final part. The resin mixture can also be used as an adhesive as is, or the resin mixture can be used as is for resin transfer molding and resin film infusion composite applications.
The effect of thermal history is usually undesirable, but inevitable during melt processing. The subject resin mixture can be melt processed by two methods, a completely homogeneous resin system or slurry mixing. The preparation of homogeneous resin requires higher temperature than slurry mixing, and therefore, more thermal history in the mixing to dissolve all the bismaleimide powder.
Another undesirable problem of the prepregs or the bismaleimide resin mixture is its instability during storage. During storage the resin mixture can continue to chemically change which manifests itself in continually shortening gel time, continually reducing pot life and reduced prepreg and adhesive tack. This is especially undesirable because the processing conditions to make the final product from the resin mixture will require adjustment to account for the changed reactivity.
As a result, there is a need in this technical area to control the advancement and chemical reactivity of the resin mixture during manufacturing, storage, and final part processing such that the subject resin mixture can be inhibited from polymerization until the cure cycle.